1. Field of the Invention
This invention relates to an electrically operated emergency system for supplying pressurized hydraulic fluid to a vehicle hydraulic steering or other mechanism which is normally supplied by an engine driven pump.
2. Description of the Prior Art
It is known to steer a vehicle by means of hydraulic actuators which are supplied with pressurized hydraulic fluid from a pump which is driven by the same engine which propels the vehicle. Steering is accomplished by utilizing the operator's steering wheel or other steering device to control the flow of pressurized fluid to the steering actuators.
It is known to provide an electrical emergency hydraulic system for such a vehicle in which there is a sensor which detects a loss of pressure or other condition of the hydraulic circuit in the vehicle steering mechanism which indicates that an emergency source of hydraulic pressure is required. Typically this is a pressure sensor which senses a loss in pressure in the hydraulic steering mechanism below a predetermined amount which closes a switch and energizes an emergency hydraulic pump driven by an electric motor.
A drawback to such an arrangement is that the pressure sensitive switch is closed when there is no pressure in the hydraulic steering mechanism and this condition obtains during starting of the engine of the vehicle which operates the main steering hydraulic pump. Thus the emergency steering system is energized when engine starting is initiated. This imposes an additional load on the electrical system during engine starting and in adverse conditions such as cold weather starting can interfere with engine starting.
It is known to provide an electrical automatic system for preventing operation of the electric motor driven pump during engine starting and such a system is shown in U.S. Pat. No. 4,079,806 Hoag et al. However, in this patent the relay which prevents operation of the emergency steering system by opening the circuit to the electric motor driving the emergency steering pump, is operated by a switch which closes each time the transmission of the vehicle is placed in or shifted through neutral. Thus the relay is subject to considerable mechanical wear and as a result may not be ready for emergency operation should that be required. The present invention avoids this deficiency.
The present invention also provides, in one aspect, an arrangement which makes it possible to operate the emergency steering system pump even though the engine of the vehicle is not running, so that the vehicle can be steered to allow it to be pushed or otherwise moved utilizing an external source of power. U.S. Pat. No. 3,940,931 Renfro et al illustrates a different emergency steering system which provides for the steering of a vehicle even though the engine is not running. In another aspect the present invention provides for testing the emergency hydraulic system while the engine is running.